Current technologies for fabricating ultra-large scale integrated-circuit devices employ copper interconnects. Interconnections using copper have replaced aluminum in the fabrication of ultra-large scale integrated-circuit devices due to its lower specific resistance and improved electromigration (EM) characteristics.
Usually, the copper interconnects are surrounded by barrier liners, such as tantalum (Ta) and/or tantalum nitride (TaN), to prevent outdiffusion and corrosion of the copper interconnect lines. For example, copper can diffuse into the surrounding dielectric materials at low temperatures, leading to device performance degradation. Copper can also be oxidized and corroded during the standard processing of device fabrication, such as oxygen or hydrofluoric acid (HF) exposure.
A damascene process can be used to form copper interconnect structures. Basically, a damascene process includes etching an interlevel dielectric layer (ILD) to form lines and via patterns, lining the patterns with barrier materials, and then filling them with copper, followed by a planarization process, e.g., chemical mechanical planarization, to remove excess copper and barrier materials.
In semiconductor devices using copper interconnects, time dependent dielectric breakdown (TDDB) characteristics can be important aspects as compared to other interconnect metals such as aluminum or tungsten, due to the high diffusion rate of copper ions. For example, after the planarization, copper can migrate over time to cause bridges between adjoining copper wirings, leading to the deterioration of leakage current characteristics.
As an example, in the basic damascene process for copper interconnect structures, there can be a ‘triple point’ where Cu/liner/ILD come together after the planarization step, which can be a weak spot for material diffusion. For example, at the “triple point”, copper ions can easily inject into the dielectric layer, since the dielectric layer can include a damaged portion due to the deposition of a cap layer. Further, there can be local diffusion of corroding substances into the interconnect line during processing. The weak spots thus can decrease operational reliability due to current leakage and thus increase the risk of operational breakdown, such as time dependent dielectric breakdown (TDDB).
In addition, high electric field can be observed at the top edges of the interconnect lines, which can cause operational breakdown or electromigration failures due to the high electrical field concentration.
It is therefore desirable to provide processes and structures of copper interconnect to improve device reliability.